1. Field of the Invention:
The present invention relates to a printed-wiring board, more particularly to a printed-wiring board comprising at least one electronic circuit and a substrate having two or more conductor layers for mounting said at least one electronic circuit.
2. Description of the Prior Art:
It is well known that a printed-wiring board mounted with a circuit element such as a transistor, an IC or an LSI often generates an electromagnetic noise, and hence a failure is sometimes caused in an electronic apparatus itself which has the built-in printed-wiring board or in other related electronic apparatus.
Among causes which generate the failure of the electronic apparatus of this like, the cause of particular large weight is a conductive or a radioactive electromagnetic wave produced by a current (a sneak current) which flows through the parasitic capacity or the parasitic mutual inductance of a circuit, called a common mode phenomenon. Since the generation mechanism of the electromagnetic wave of this kind is complex, an effective countermeasure near the failure generation source which is considered most effective has not been established. Therefore, a conventional countermeasure has been to control the conduction or the radiation of the electromagnetic wave by electromagnetically shielding the whole electronic apparatus with a metallic cover and providing a common-mode choke coil or core for the cable outwardly extending from the apparatus. On the other hand, with reference to the printed-wiring board of the conventional type, as shown in FIG. 2, high frequency capacitors C1-Cn are often connected between power supply terminals 3a-3n and ground terminals 4a-4n of electronic circuits Z1-Zn such as the IC, LSI (hereinafter simply called an electronic circuit) connected respectively to power supply line 1 which corresponds to power source conductor layer 1 and grounding line 2 which corresponds to ground conductor layer 2 of the printed-wiring board. The purpose of this arrangement is to flow the high frequency current, which flows in power supply line 1 accompanying the high frequency operation of electronic circuits Z1-Zn, in high frequency capacitors C1-Cn to decrease (that is, filtering) the high frequency fluctuation of the power source terminal voltage and to restrict the coupling (that is, decoupling) with other electronic circuit sharing power supply line 1. However, in particular, for effectively reducing the fluctuation of the power supply voltage, the conventional printed-wiring board is structured, as shown in FIG. 1, with power source conductor layer 1 made of all flat plate having no wiring in order to lower the impedance of the power supply line. The conventional printed wiring board is structured, as shown in FIG. 1, with insulative board layers 6, which are, e.g., typical resin substrates.
The above conventional printed-wiring board has a problem that a circuit designer can not control the high frequency current which flows in the power supply line accompanying the high frequency operation of said electronic circuit.
In other words, when the power conductor layer is made of a flat plate over the whole area without having wiring, the impedance of the power supply line is small, and hence even if a high frequency capacitor is connected between the power source terminal and the ground terminal of the electronic circuit, it is hard to control the high frequency power source current which flows out to the power supply line, and resultantly the high frequency power source current flows in the decoupling capacitor disposed in the vicinity of other electronic circuits which share the power supply line. Therefore, the analysis of the high frequency power source current with reference to the whole printed-wiring board is very difficult and hence it is unable to strictly select the decoupling capacitor to be used for each electronic circuit.
Further, the distribution of the high frequency power source current on the whole printed-wiring board is very complex. Depending on the arrangement of electronic circuits-on the printed-wiring board or on the operation state of the electronic circuits even if they are in the same arrangement, the high frequency power source current distributes in such a manner to form a large loop, having the probability of introducing the problem of electromagnetic radiation or immunity.
For example, as shown in FIG. 2, electronic circuits Z1 (a large current), Z2 (a medium current) and Zn (a small current) for different high frequency power source currents are connected respectively to both power supply line 1 and ground line 2. Mainly from the mounting and the cost restrictions, capacitors C1, C2 and Cn of capacitance corresponding to respective high frequency power source currents (large capacitance for Z1, medium capacitance for Z2, small capacitance for Z3) are connected to respective electronic circuits. When the electronic circuits on the printed-wiring board operate in this state, since capacitor Cn of small capacitance, connected between the power source terminal and the ground terminal of electronic circuit Zn for small high frequency power source current, has the impedance larger than that of other capacitors, the high frequency power source current of electronic circuit Zn flows into capacitor C2 of medium capacitance (having small impedance compared to that of Zn) connected between the power source terminal and a ground terminal of electronic circuit Z2 for a medium high frequency power source current, or into capacitor C1 of large capacitance connected between the power source terminal and the ground terminal of electronic circuit Z1 for a large high frequency power source current. Further, if the capacitors are of the same type, the frequency characteristic of the impedance becomes different according to each capacitance, and the impedance of the capacitor of small capacitance tends to become smaller than the impedance of the capacitor having the larger capacitance, in the region where the number of the high frequency exceeds some value. Therefore, when viewed in the broad frequency band, electric currents flow to and fro in a complicated way between electronic circuits on the printed-wiring board. As a result, the loop made by the high frequency power source current becomes large or the high frequency current of the common mode is made to flow into such as the cable connected from one printed-wiring board to another printed-wiring board, thereby increasing the conductive or radioactive electromagnetic waves.
These phenomena have caused the electromagnetic interference between the electronic apparatus, that is, EMC problem. Further, for example, high frequency power source voltage fluctuation with reference to one electronic circuit on the printed-wiring board sometimes becomes an issue. In this case, even if the capacitance of a decoupling capacitor in the vicinity of the electronic circuit is increased, it is not effective because the more the capacitance is increased, the more the high frequency power source currents of other electronic circuits flow into the circuit, and consequently it becomes hard to decrease the high frequency power source voltage fluctuation with the probability of failing in securing the high frequency operation of the electronic circuit.
Further, when the latest IC/LSI is used in a logical unit which operates at a low frequency, for preventing the generation of the unnecessary conductive or radioactive electromagnetic waves to be caused due to excessively fast rise time or fall time of a rectangular wave signal of the IC/LSI, sometimes a high frequency filter is inserted in the circuit to slow the rise time and the fall time of the rectangular wave signal of the IC/LSI. However, when the high frequency filter of this like is newly added as parts, the high density mounting efficiency on the printed-wiring board is disadvantageously affected.
Therefore, with the conventional printed-wiring board, it becomes necessary to electromagnetically shield the whole electronic apparatus with metal foil, as described above, to protect against the EMC problem and concurrently control the conduction of the electromagnetic waves by attaching a common-mode choke coil or core for the cable extending outwardly of the apparatus. Still further, with reference to the problem which hinders the high frequency operation of the IC/LSI, it is necessary to meet the problem by mounting capacitors of a required number or more on the printed-wiring board to control the high frequency power source voltage fluctuation of the whole wiring board.